1. Field of the Invention
This invention relates to compositions of transition metal dichalcogenides for storing hydrogen and gaseous hydrocarbons and methods for preparation thereof.
2. Description of Related Art
There has been intense interest and considerable research activity directed to ways for storing hydrogen gas other than the conventional manner as a highly compressed gas. The cylinders used to store the compressed gas are heavy, costly and potentially dangerous because of the high pressure of flammable gas they contain. One reason for the great activity in pursuing a better storage system is the potential use of hydrogen as an ecologically sound substitute for gasoline as a vehicle fuel. Prototype vehicles burning hydrogen fuel have already been produced, but one of the factors delaying production is the lack of an acceptable storage means. Preferably, the storage means should also be suitable for gaseous hydrocarbons as well.
Materials have been developed which aid in storing such gases at relatively low pressures. Desirable characteristics of suitable materials include (a) the ability to adsorb or absorb large quantities of the gas in a minimum volume, (b) provision for easy access of the gases to the adsorption or absorption sites, and (c) easy reversibility so that the gas can be reclaimed from the storage material easily and conveniently.
One approach has been to use materials which form hydrides which are easily decomposed. See, for example, U.S. Pat. No. 4,948,423 and 5,002,730 to Fetcenko as well as U.S. Pat. No. 4,946,646, 4,923,770, 4,915,898 and 5,006,328. Examples of such materials are LaNi.sub.5, CaNi.sub.5, FeTi, Mg.sub.2.4.Ni and CoNi.sub.5. These materials absorb hydrogen, allowing large quantities of the gas to be stored. Each of the above compositions covers a certain range of pressure and temperature. For example, LaNi.sub.5 performs well between 1-2 atm at 20.degree. C., forming the hydride of LaNi.sub.5 H.sub.6. CaNi.sub.5 is a low pressure hydride, below 0.5 atm. FeTi operates at 10 atmospheres and Mg.sub.24.Ni works at high pressures of 10 atm and at high temperatures--more than 300.degree. C. The gas is reclaimed by heating the cylinder or other container.
Many of such materials do not provide satisfactory storage and release of the gas. There is simply no way in which the gas has access to the interior of such materials to react therewith. In order to provide pores through which the gas can move to and from the absorption sites, the absorbent is typically supported by a material, such as alumina particles, that have no role in the absorption process as such, and take up valuable space as well as adding weight. The alumina particles are typically pelletized into small pellets of approximately 1 cm.sup.3, to permit gas movement. The gas can move between the pellets freely, and with the small pellets can enter the pores of the pressed alumina and reach the absorbent.
The production of single molecular thickness layers of MoS.sub.2 and other transition metal dichalcogenides is disclosed in U.S. Pat. No. 4,822,590 to S. Roy Morrison et al. The MoS.sub.2 is exfoliated into monolayers by intercalation with lithium followed by reaction with water. The reaction between the water and the lithium forces the layers of MoS.sub.2 apart into one-molecule-thick platelets. Flocculation occurred rapidly when the pH was reduced to a value of 3 or less.
U.S. Pat. No. 4,996,108 to Divigalpitya et al. carries this process further by forming a suspension of a transition metal dichalcogenide in water and adding to the suspension a liquid which is immiscible with water. The mixture is agitated, then allowed to rest. A sheet-like composition of about 500 .ANG. to 750 .ANG. forms at the interface of the water and the immiscible liquid.
In U.S. Pat. No. 4,853,359 to Morrison et al., a novel flocculated, supporting single-layered transition metal dichalcogenide catalyst is disclosed. The patent discloses that if the pH is maintained at a value about 3, the suspension of MoS.sub.2 will not flocculate, but will stay four days or more suspended in water. The next step in producing the catalyst according to the patent is to add a catalytic promoter to the suspension. A solid is formed by adding this promoter or support substance to the water. The flocculated solid is then separated from the liquid.
In an Article, J. Appl. Phys. 69(9) 6373 (1991) by Miremadi et al., the deposition of thick oriented films of a transition metal dichalcogenide, with a thickness in the order of 0.1 to 10 microns, is disclosed.
In pending U.S. patent application No. 07/704,432, a novel transition metal dichalcogenide with a house-of-cards (HOC) structure is disclosed and method for production thereof. The method involves reducing the pH of a suspension of single molecular thickness platelets of MoS.sub.2, or other transition metal dichalcogenides, until they flocculate, and the pH is within a range between a first pH where basal planes of the platelets have a zero charge and a second pH where edges of the platelets have a zero charge.